This invention relates generally to a packaging method and machine for wrapping stretch film around products supported on trays, and more particularly to a package wrapping method and machine employing an improved film gripper having sections which may be selectively disengaged to accommodate wrapping packages of smaller sizes and a package pusher which more consistently deposits a tray to be wrapped at a known registration position on an edge of a package elevator.
A variety of machines are available for wrapping film around trayed products, for example meats and produce in supermarkets, to prepare attractive packages for consumer display. An early example of such a machine is illustrated in U.S. Pat. No. 3,662,513 which discloses using a single length and width of stretchable film for a narrow range of package sizes and heights. These prior art machines would have to be shut down for making several manual adjustments if another range of package sizes was to be wrapped. When wrapping packages in this design, in order to properly wrap the largest or highest package within the range, excess and therefore wasted film results for the smaller sizes. Excess film creates unsightly clumps and tails of film on the bottom of such smaller packages, and this in turn results in poor bottom seals and leaking packages.
To overcome the problems created by using a single fixed length stretchable film sheet for wrapping several package sizes, film wrapping machines have been developed which permit the automatic selection of a variety of film lengths to accommodate varying package sizes. An example of an automatic film length selection mechanism for a film wrapping machine is disclosed in U.S. Pat. No. 4,510,731. In this patent, the automatically measured length of a package controls the stroke distance of a reciprocable film gripper to draw the appropriate length of film from the supply roll for wrapping the particular package. However, the stroke adjusting mechanism is complicated, requiring special mechanical linkage, an adjusting control motor, sensing switches and control electronics to automatically vary the lengths of film drawn into the machine. Accordingly, while the adjustable mechanism was a substantial improvement over the fixed film length wrapping of then-existing prior art, it entailed greater initial cost and maintenance expense.
Another film sheet sizing arrangement is disclosed in French Patent Publication No. 2,410,601 wherein a film sheet is held across the path of an obliquely raised elevator. In the French publication, a package is placed on the elevator and obliquely raised into and through a plane defined by the film to thereby draw any additional film which is required by the height of the package from a continuous source of the film. The obliquely raised elevator comprises a surface which is sequentially replaced by a film underfolder to thereby support the package and permit the elevator to be lowered for the next package to be wrapped. Film length is determined by severing the sheet only after the underfolder has been entirely inserted under the package to position the severed film end at a film holding mechanism for the next package. The leading and trailing edges of the film are brought together to form an overlapping sleeve arrangement for wrapping the package.
Another film sheet sizing arrangement is disclosed in U.S. Pat. No. 4,813,211, issued Mar. 21, 1989, entitled "Package Wrapping Method and Machine," to Treiber and assigned to the same assignee as the present application. According to this patent, the length of each package is measured during its travel to a wrap station. A fixed length of film is initially drawn by a film gripper into the wrapping machine to the same stop or end position for all package sizes. The package is placed on an elevator and is raised into and through a plane defined by the film, thereby permitting the package height to draw a first film addition from the source if the height is above a certain minimum. A rear underfolder bar is moved horizontally into contact with the film at a location between the package and the film source to commence film underfolding beneath the package. As this takes place, a heated cut-off wire is passed through the film vertically, behind the bar. The timing of the wire passage is such as to provide the proper film length in accordance with the initial measured length of the package. The net result is to provide an initial fixed film length for all package sizes, a first inherent additional film draw for high packages and a second measured film length by timing an on-the-fly sheet cut to occur after the package is fully elevated and has pulled all the extra film needed. The first additional film draw is made somewhat similarly to that of the aforementioned '601 French Publication. However, a problem encountered in the '211 patent system occurs when using heavy gauge film or using foam trays with weak edges. Under such circumstances the film tension force applied to the foam trays at their trailing edges caused trays to break or crack on occasion during underfolding. That requires a rewrap and results in lost production time. In addition, because the rear underfolder of the '211 patent is also used to advance the tail of the film for obtaining the second featured length, it is necessary to use a cut-off or severing means capable of effectively cutting "on-the-fly" and passing through the moving film rapidly while it is under tension. Not only is a mechanical knife felt unsuitable for cutting moving film, but an on-the-fly cut requires immediate braking of the film and roll behind the cut, in order to properly present the cut end to film grippers for the next sheet of film to be pulled.
Yet another patent, U.S. Pat. No. 5,144,787 to Whitby et al., the inventor of the present invention and assigned to the assignee of the present invention, discloses a package wrapping method and machine with improved features for paying out the required amount of stretch wrap from the supply roll for each individual package. In the '787 patent, a tray containing goods to be wrapped is placed on an infeed conveyor where one of a plurality of package pushers on a continuous loop receives the package and carries it to an elevator. The pusher is braked just short of the elevator depositing the package near the front edge of the elevator, and the pusher is maintained in this position until the elevator is raised sufficiently to allow clearance for the pusher as it continues on the loop. Simultaneously, after film from an appropriate supply roll is chosen based on a premeasured package width, a fixed amount of stretch film is drawn horizontally from the selected supply by a gripper over the elevator opening. Next, the elevator is raised through the film, and the package draws a first additional length of film from the roll based on the height of the package. Then, if required, a second additional length of film is drawn from the roll by a vertically moving horizontal bar located adjacent the roll, based on the premeasured package length. Finally, the film is cut and folded around the package and the package is transported from the machine by a heated belt which seals the overlapping edges of the film under the tray.
While the above-identified inventions solve some of the problems associated with wrapping packages of varying sizes, several other problems have not been sufficiently addressed prior to the wrapping machine of the present invention. For one, due to the precision required for the wrapping procedure, it is essential for the package to be properly positioned at all times during the wrapping process. Since the package's dimensions are measured at the beginning of the process and the location of the pusher is known, if no other significant variables are introduced, the location of the package at any point in time can be determined. However, the braking of the pusher as it approaches the elevator as described in the '787 patent affects the final location of the package in that the package has a tendency to slide on the elevator due to the momentum produced by the pusher. The variables that contribute to this sliding effect can be difficult to account for and include the speed of the pusher, the weight of the package, and the coefficient of friction of the tray, among other things. The consequences of not having the package consistently located at the same position on the elevator includes finished packages that are not adequately sealed, as well as possible package jams which may lead to lost product, equipment failures and equipment down time.
Furthermore, another problem not effectively addressed by prior art wrapping machines involves the production of excess wrapping material in the form of unsightly "tails" which extend from the bottom of smaller wrapped packages. The reason for this occurrence is that the lateral sections of the film held by the gripper during the folding process become stretched due to the tension placed on the film. With larger packages, this poses very few problems since the additional film can be neatly tucked and sealed under the tray. However, for a very small package, even when the narrowest roll of film is selected from the roll supplies, the width of the film is still sufficiently greater than the width of the package that it cannot all be folded under the package, especially given the degree that the film is stretched during the folding process. Not only do these tails detract from the overall appearance of the wrapped package, they also prevent the creation of a good seal around the wrapped product, threatening the integrity of the goods therein. While it is conceivable that these tails could be greatly reduced by offering a larger selection of varying width stretch film supply rolls for the gripper, equipment costs and design complexity, as well as other design constraints, make this solution undesirable.
Accordingly, there exists a need for an improved package wrapping machine which consistently produces a wrapped product having a good seal and a favorable appearance for packages of various sizes, which is of relatively simple design, and which is efficient and reliable in operation. Preferably, these improvements will be obtained by a wrapping machine which locates the package to be wrapped in a predetermined position on the elevator, which eliminates package "slide" on the elevator, which alleviates some of the stretching of the film during the folding process for smaller packages thereby diminishing the size of the tails protruding from the bottom of the wrapped package, and which is extremely user-friendly requiring minimal operator inputted information or skills. Most preferably, portions of the improved design features will be retrofittable to prior art machines thereby allowing the new improvements to be implemented in old machines without the need for purchasing all new equipment.